1. Field of the Invention
The present invention relates to a magneto-resistive effect (MR) element and particularly to a configuration of a bias magnetic field application layer.
2. Description of the Related Art
A MR element used for a hard disk drive (HDD) or the like has a magneto-resistive (MR) stack provided with a magnetization free layer and configured such that a sense current flows therethrough. The magnetization free layer is a magnetic layer of which a magnetization direction changes according to an external magnetic field. The MR stack further has a magnetization pinned layer of which a magnetization direction is pinned, and a spacer layer positioned between the magnetization pinned layer and the magnetization free layer. These three layers change electrical resistance of a sense current based on a MR effect. In another case, the MR stack may have two magnetization free layers and a spacer layer positioned between these magnetization free layers. Also in this case, three layers—the two magnetization free layers and the spacer layer—change electrical resistance of a sense current based on the MR effect. The MR stack reads magnetic data recorded on a magnetic recording medium from the electrical resistance change.
Even with either of the configurations, in order to obtain a large MR change, the magnetization free layer preferably forms a single magnetic domain in a state where no external magnetic field is applied. Accordingly, bias magnetic field application layers are provided on sides of the MR stack, specifically sides of the magnetization free layer, to let the magnetization free layer form a single magnetic domain. The bias magnetic field application layers are mainly configured with hard magnetic layers, and apply a bias magnetic field toward the sides of the MR stack in a direction perpendicular to a lamination direction of the MR stack.
In conjunction with an increase in a recording density of a HDD, it is required for a hard magnetic layer to have a high saturation magnetization (Bs) and high coercive force (Hc). A high saturation magnetization helps to intensify a bias magnetic field and to stabilize a magnetization direction of a magnetization free layer that is narrowed due to the narrowing of tracks. High coercive force enhances the magnetization stability of the hard magnetic layer. Specifically, even when, due to the narrowing of the lead gap, effects of magnetic function from upper and lower shield layers increase, a magnetic tolerance of the hard magnetic layer is enhanced. When, a flying height of a magnetic head slider is reduced for the increase in the recording density of a HDD, a contact tolerance with a magnetic recording medium is enhanced.
Using a FePt alloy for the hard magnetic layer has been known for enhancing the coercive force and the saturation magnetization of the hard magnetic layer. A high temperature annealing is normally necessary to regularize the FePt alloy. However, performing the high temperature annealing causes dispersion of magnetization directions of other ferromagnetic layers, for example, the magnetization pinned layer of the MR stack. Therefore, it is preferable to cause the regularization with an even lower temperature. U.S. Application Publication No. 2009/0274931 and U.S. Application Publication No. 2010/0047627A1 disclose a configuration of such bias magnetic field application layer. The bias magnetic field application layer is provided with a seed layer and a cap layer, and an intermediate alloy layer (hard magnetic layer) positioned between the seed layer and the cap layer. The seed layer and the cap layer can be formed of Pt. The intermediate alloy layer is formed with a plurality of FePt layers, and the FePt layers adjacent to each other have different compositions.
Because Pt is conductive, a direct contact of the seed layer made of Pt with the MR stack may cause a bypass path for a sense current that connects the MR stack with the hard magnetic layer via the seed layer. Similarly, a direct contact of the seed layer made of Pt with a lower shield layer may allow a sense current to flow from an upper shield layer to the lower shield layer through the hard magnetic layer. These phenomena interfere with a sense current focusing on the MR stack. Accordingly, an insulation film is formed between the hard magnetic layer and the MR stack and between the hard magnetic layer and the lower shield layer. Upon a manufacturing process, the seed layer is formed not only under the hard magnetic layer but also on the side of the MR stack. Due to this, the bias magnetic field application layer is insulated from the MR stack and the lower shield layer, and a sense current flows focusing on the MR stack. Al2O3 is normally used for the insulation film.
However, Al and Pt are more likely to mix with each other and Pt diffuses into Al2O3, and thereby the insulativity of Al2O3 may decrease or lose. In that case, a sense current partially flows to the bias magnetic field application layer, and a sense current may be inhibited from focusing on the MR stack. Forming a diffusion prevention layer between the insulation film and the seed layer allows to prevent Pt from diffusing into Al2O3; however, because the diffusion prevention layer is formed on the side of the MR stack, a distance between the MR stack (especially the magnetization free layer) and the bias magnetic field application layer increases, and the intensity of a bias magnetic field applied from the bias magnetic field application layer to the magnetization free layer decreases.
An objective of the present invention is to provide a MR element which can be annealed with a low temperature and with which a focusing of a sense current on the MR stack is easy.